Sensor devices are known which can monitor bioelectric data from a body. For example, B J ten Voorde in "High resolution magnetic mapping of PR-interval phenomena of normal subjects", Med. & Biol. Eng. & Comp., 26, 130-135, 1988 has reported the measurement of magnetocardiograms (MEGS) using SQUID detectors achieved by a gradiometer with fixed grid spacing of 5 cm and a total grid dimension of 25 by 30 cm. Cullen and Dempsey in "NIBEC ECG mapping harness", Proc. 14th Ann. Int. Conf. EEE Eng. in Med. & Biology Society, Pt 6, p. 2702-2703, October 1992 have reported the utilization of a fixed grid harness for body surface mapping of electrocardiograms (ECGs). Barber et al. "Extensions to the Sheffield filtered back projection algorithm to reconstruct any bipolar drive data", Proc. 15th Ann. Int. Conf. EEE Eng. in Med. & Biology Society, Pt 1, p. 78-79, 1993 have reported the use of a single row of sensor sites to perform slice imaging of applied potential tomography or electrical impedance tomography. These examples highlight some of the current techniques in obtaining body surface data. The need to extract more diagnostic information is now proving beneficial as with two dimensional and three dimensional imaging such as in X-ray and magnetic resonance imaging.
In particular, ECG body surface mapping has been shown by F. Konreich et al., "Body surface potential mapping of ST segment changes in acute myocardial infarction", Circulation, 87, 3, 1993 to improve the detection of acute myocardial infarction. This imaging modality is offering powerful means for characterizing and assessing abnormalities of myocardial muscle. In detecting acute myocardial infarction the standard 12 lead ECG which provides one of the most important first-line assessments can be often equivocal (M. Hirai et al., "Body surface isopotential maps in old anterior myocardial infarction undetectable by 12-lead electrocardiograms", American Heart Journal, Vol. 108, No. 4, Pt 1, p.975-982, 1994. Improved detection in these cases will inevitably result in benefit to the patient through quick and reliable selection of those most likely to respond to thrombolytic therapy.
One system which has reported improved sensitivity in detecting acute myocardial infarction is disclosed in British Patent Specification No. GB 2 264 176 and its counterpart U.S. Pat. No. 5,419,337, issued Jul. 14, 1995. In that specification, there is disclosed an apparatus for the detection, recording and analysis of the electrical activity of a cardiac muscle. The apparatus comprises an array of a plurality of n number of sensors where n is an integer from 40 to 100 each of which is capable of detecting an electrical signal associated with the Q and/or ST components of a heartbeat. The array is connected to a microprocessor controlled interface which in turn is connected to a microprocessor controlled analyzer and display apparatus. Each sensor provides an independent electrical picture of the heart when it is contracting as each one detects the summation of electrical changes in the heart from different angles or notional slices. Injury to the heart causes distortion or disturbance to this electrical activity and will result in changes in the potential pattern of those sensors providing information on the injured area. By means of suitable electronic systems, there is then presented a three-dimensional profile or ST map of the ST level processed from each sensor lead. The sensor array shown in the specification referred to comprises a two-dimensional array of a plurality of sensors of well-known construction.
There have been various attempts to configure sensor arrays to the thoracic surface of a human body. Dempsey et al. in "Diagnosing Myocardial Infarction using a portable Cardiac Mapping System", CEC, 2nd Eur. Conf. on Biomedical Engineering; Vol. 1, p.222-223, 1993 reported on a harness which utilizes a fixed grid array of sensors screen printed onto a flexible substrate and utilizing hydrogel sensors to enable rapid application. The Corazonix Predictor BSM-32 array marketed by Corazonix Corporation, Oklahoma, USA relies on marking the thorax into segments using a marker pen and then placing individual columns of gelled sensors into the vertical grids. Hirai et al. in "Body surface isopotential maps in old anterior myocardial infarction undetectable by 12-lead electrocardiograms", American Heart Journal, Vol. 108, No. 4, Pt 1, p. 975-982, 1984 have also reported the use of 87 sensors placed in a grated system composed of 13 columns and 7 rows on the thorax. The problem remains that to apply in excess of 20 sensors onto the thorax with anatomical consistency requires long periods of application time and is only practical under ideal clinical conditions.